docs(utils/sessions): improve docs

[aaronadamsCA]

See #7225 (comment) for details. In the absence of a larger rewrite I think this should substantially improve clarity and reduce confusion.

Key bits include:

• Clarifying that regardless of your session storage strategy, if you don't commitSession(), you're losing your changes.
• Further clarifying that if you are using cookie session storage and you don't Set-Cookie, you're still losing your changes.
• Correcting the cookie size limit. It's actually 1024 bytes for a single cookie value in Chromium and Firefox (sources: RFC 6265, Chrome Platform Status). 4 KB is the limit for all cookie names + values combined.
• Consistently recommending app/session.server.ts over app/session.ts.

The one more opinionated thing I did as well was to remove the lengthy example under session.flash(), because there is already an example of the same thing earlier in the doc.

Please let me know if there's anything else I can do to help ship this, so that I can stop making the same mistakes over and over every time I read this doc page. Thanks!

@brophdawg11

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[remix-cla-bot]

Thank you for signing the Contributor License Agreement. Let's get this merged! 🥳

[MichaelDeBoey]

@aaronadamsCA Are you still interested in getting this PR merged?
If so, could you please rebase onto latest main & resolve conflicts?

[aaronadamsCA]

Whoops, thanks for the bump, @MichaelDeBoey! I completely forgot about this.

I've updated the branch and made @brophdawg11's requested changes. The only thing that needs checking is the line numbers in the code block, I'm not sure how to preview those to ensure the correct lines are being highlighted.

[aaronadamsCA]

Thanks for the merge @brophdawg11, though as I read these changes back a year and a half later, I desperately hope React Router middleware can at last solve all these race-condition pains. 😅